1. Field of the Invention
This invention relates to a leak detection method for electrical equipment provided with an electric motor.
2. Description of the Prior Art
FIG. 10 shows an electric circuit for electrical equipment or, for example, a washing machine. A feeder circuit for a washing machine motor 1 will be described. A DC power supply circuit 4 is connected via a power switch 3 to a commercial AC power source 2. The DC power supply circuit 4 comprises a voltage doubler rectifier circuit 8 composed of a diode bridge 5 and capacitors 6 and 7 connected as shown in the figure, and a voltage regulator circuit 9. An inverter main circuit 10 is connected to an output side of the DC power supply circuit 4 and comprises six switching elements 10a to 10f. The motor 1 includes phase windings 1u, 1v and 1w all connected to the inverter main circuit 10. A control circuit 11 comprising a microcomputer controls a switching element drive circuit 12 further connected to the switching elements 10a to 10f so that the switching elements are turned on and off. The control circuit 11 further controls a water-supply valve 13, a drain valve, etc.
On the other hand, a zero-phase-sequence current transformer 16 is provided on a feed path between the AC power source 2 and the DC power supply circuit 4. The transformer 16 delivers an output signal via a detecting resistance 16a to a leak detection circuit 17. The zero-phase-sequence current transformer 16 and the leak detection circuit 17 constitute a leak detector 18. The leak detection circuit 17 comprises an amplifier circuit, a reference voltage generating circuit and a comparator circuit.
When leak occurs in the motor 1 during energization of the motor, only an amount of current obtained by subtracting a leaked current from a current supplied from the AC power source 2 is returned to the source. Accordingly, an amount of positive current is unequal to an amount of negative current, whereupon a detection current is produced in the transformer 16. When the detection current is delivered as a corresponding detection voltage from the detector resistance 16a, the voltage is amplified by the leak detection circuit 17 and further compared with a reference value, so that the presence or absence of the leak is detected.
In the above-described arrangement, however, the leak detection circuit 17 of the leak detector 18 consumes a large amount of power. Further, the leak detection circuit 17 necessitates a high precision and high gain amplifier circuit since it detects a slight unbalance of load current. Such an amplifier circuit increases the cost. Additionally, leak can be detected only during energization of the motor 1, namely, the motor 1 needs to be actually driven for the leak detection.
Therefore, an object of the present invention is to provide a current leak detection method for a washing machine which can reduce the power consumption and eliminate the high precision circuit to thereby reduce the cost, and in which leak can be detected without driving the motor.
The present invention provides a current leak detection method for a washing machine provided with an electric motor, an inverter circuit driving the electric motor and a voltage doubler rectifier circuit supplying driving power to the inverter circuit and including two capacitors having a neutral point connected to a grounded side of an AC power supply, the voltage double rectifier circuit having a ground potential set to be equal to a ground potential of an internal circuit of the washing machine. The washing machine including an outer cabinet having a potential set at a value differing from a ground potential of an internal circuit thereof, the method comprising detecting, during non-energization of the motor, a voltage generated in a voltage-dividing resistor provided between a winding of the motor and ground on the internal circuit, thereby detecting leak produced in the motor.
According the above-described method, no voltage is applied to the motor winding when no leak occurs between the motor and the outer cabinet of the washing machine during non-energization thereof. Accordingly, no voltage is generated in the voltage-dividing resistor. On the other hand, when leak occurs between them during non-energization of the motor, a potential difference between the outer cabinet and a ground on the internal circuit causes current to flow from the motor winding to the resistor such that voltage is produced in the resistor. Accordingly, the voltage produced in the voltage-dividing resistor is detected during non-energization of the motor, whereby leak produced in the motor can be detected. Since the leak can be detected by such a simple and less expensive arrangement that the voltage-dividing resistor is provided, the cost can be reduced and power consumption can be reduced. Additionally, leak can be detected without driving the motor.
In a first preferred form, the leak detection is carried out after the washing machine is connected to a power supply and before the motor is operated. Since the leak is detected before operation of the motor, the safety can be improved. In a second preferred form, the motor is energized from a DC power supply obtained by converting and AC power supply, the method further comprising determining whether the voltage generated in the voltage-dividing resistor has, a dc waveform or a pulsating waveform, with the outer cabinet of the washing machine being connected to a neutral point of a capacitor dividing the AC power supply, setting a reference voltage according to a result of determination, and comparing the voltage produced in the voltage-dividing resistor with the reference value, thereby detecting the leak.
Generally, when the user uses the washing machine, an outer cabinet of the washing machine is grounded by the user for prevention of electrical shock. When a power supply for the motor is energized from a DC power supply obtained by converting an AC power supply, the motor power supply is a stable direct current. When leak occurs in the washing machine, voltage having a direct current waveform is detected in the voltage dividing resistor. Further, when the outer cabinet of the washing machine is not grounded, the motor power contains the pulsating waveform since the outer cabinet of the washing machine is connected to a neutral point of a capacitor dividing the AC power supply. Upon occurrence of leak in this state, voltage having the pulsating waveform is produced in the voltage-dividing resistor.
Moreover, the dc waveform and the pulsating waveform have different correspondences to the actual leak current. That is, when the leak currents are equal to each other between the dc waveform and the pulsating waveform, the voltage having the dc waveform is higher than the voltage having the pulsating waveform. Accordingly, in a case where the leak detection is carried out with the same reference voltage, it is determined that leak has occurred, when the generated voltage has the dc waveform, though it is determined that no leak has occurred, when the generated voltage has the pulsating waveform.
In the above described method, however, it is determined whether the voltage generated in the voltage-dividing resistor has, the dc or pulsating waveform. The reference voltage is set according to the result of determination. The generated voltage is compared with the reference voltage so that leak is detected. Consequently, the occurrence of leak can reliably be detected whether the outer cabinet of the washing machine is grounded or not.
In a second preferred form, the leak detection method further comprises connecting a ground detecting circuit having a series circuit of a switching element and a resistor between an electrical equipment housing or an earth wire provided on the housing and a power input side of the winding, turning on the switching element to determine which voltage generated in the voltage-dividing resistor has, a dc waveform or a pulsating waveform, and warning when the voltage has the pulsating waveform.
Whether the user has grounded an electrical equipment housing can be determined by determining which one of the dc waveform and the pulsating waveform the voltage generated in the voltage-dividing resistor has. However, this is on the assumption that leak has already occurred. On the other hand, in the invention, the ground detecting circuit is connected between the electrical equipment housing or the earth wire provided on the housing and the power input side of the winding. A suspected leak can occur when the switching element is turned on. It is determined which voltage generated in the voltage-dividing resistor has, the dc waveform or the pulsating waveform. Consequently, even when leak has not actually occurred, the user can determine whether the electrical equipment housing is grounded. Further, since the warning is carried out when the voltage has the pulsating waveform, the user can be informed of an ungrounded state of the electrical equipment housing.